U.S. President Barack Obama has confirmed the death of American hostage Abdul-Rahman Kassig, saying the aid worker was killed "in an act of pure evil'' by Islamic State militants.

In a statement to reporters aboard Air Force One Sunday, Obama offered "prayers and condolences" to Kassig's parents and family. He accused the IS group of reveling in the "slaughter of innocents, including Muslims, and is bent only on sowing death and destruction.''

The president was on his way home after attending the G20 summit in Australia.

Confirmation of Kassig's death came after a review of a video released Sunday by Islamic State that purported to show extremists beheading several Syrian soldiers, and concluded with a militant claiming to have killed the American.

The video did not show the beheading.

The militant in the clip, who speaks with a British accent, says the first "first American crusader" will be buried in the northern Syrian town of Dabig, and warns that U.S. soldiers will meet a similar fate.

The same video showed the beheadings of at least 18 men described as Syrian military personnel.

Kassig, a 26-year-old former U.S. Army Ranger, was taken captive 13 months ago while doing humanitarian work in war-torn Syria. His family said he converted to Islam while in captivity and changed his first name from Peter to Abdul-Rahman.

With Kassig's death, the Islamic State terrorist group has killed five Westerners it was holding. The militants earlier victims were British aid workers David Haines and Alan Henning, U.S. reporter James Foley and American-Israeli journalist Steven Sotloff, who were all beheaded.

Kassig's mother, Paula, recently told NBC's Today show that she and her husband received an audio recording of their son several weeks ago in which he said he feared his time was running out.

In a statement Sunday, the Kassig family urged the media not to broadcast images released by the militants, saying this was "playing into the hostage takers hands."

British Prime Minister David Cameron said he was horrified by Kassig's "cold-blooded murder."